Electronic circuits and particularly those fabricated in bipolar integrated circuit form often require a current supply or source capable of providing a current of substantially constant magnitude at an output terminal thereof even though the voltage at the output terminal is changed by the electrical load connected thereto. The output impedance of such a current source may be defined as the change in the magnitude of the voltage at the output terminal divided by the consequential change in the magnitude of the current supplied by the source. Ideally, if the output impedance of the current source was infinite, any change in the magnitude of the voltage at the output terminal would produce substantially no change in the magnitude of the current supplied to the output terminal. Prior art current sources suitable for fabrication in bipolar integrated circuits have finite output impedances.
One prior art current source configuration includes a series circuit connected between supply terminals which has a diode with one electrode connected at a junction to a resistor. A transistor has a base electrode connected to the junction and an emitter electrode connected to the same supply terminal which is connected to the other electrode of the diode. The collector electrode of the transistor is connected to the output terminal of the current supply.
The diode tends to clamp the base-to-emitter voltage of the transistor and to thereby provide a minority carrier density at the edge of the base of a fixed magnitude. As the collector-to-base junction voltage of the transistor changes in response to the magnitude of the voltage at the output terminal changing, then the width of the depletion spread about the collector-to-base junction changes or is modulated. This change in depletion spread produces variation in the effective width of the base region of the transistor. Consequently, the carrier gradient, which is the ratio or derivative of the minority carrier density to effective base width, in the base region changes. Since the magnitude of the collector current varies inversely with the effective width of the base region, the magnitude of the collector current changes with variation of the magnitude of the collector-to-base voltage. The base width modulation also affects the carrier recombination rate in the base region because the recombination rate is proportional to the base width. This change in recombination rate causes a generally undesirable change in the magnitude of the base current which changes the magnitude of the collector current of the transistor as a result of the change in the magnitude of the voltage at the output terminal of the current supply. Consequently, as the collector-to-base voltage is increased, for instance, the gradient is increased and the rate of recombination is decreased both of which tend to cause the magnitude of the collector current to undesirably increase and both of which thereby lower the output impedance of the supply.
An improved prior art current supply utilizes an additional transistor cascoded with the current supply transistor of the above described supply wherein the emitter electrode of the additional transistor is connected to the collector electrode of the current supply transistor. The base electrode of the additional transistor is connected to a terminal providing a reference potential. The collector electrode of the additional transistor is connected to the output terminal of the current supply.
The additional transistor tends to buffer or isolate the base-to-collector junction of the current supply transistor from changes in the magnitude of the voltage at the output terminal of the supply. Consequently, the current supply transistor provides a substantially constant emitter current to the additional transistor which tends to eliminate the change in the minority charge gradient in the base region of the additional transistor and the consequential effect thereof on the magnitude of the collector current. Thus, the improved current supply has a higher output impedance than the previously described current supply.
However, the recombination phenomenon still causes the base current of the additional transistor to change as the magnitude of the voltage across the collector-to-base junction of the additional transistor changes the base width thereof. The emitter current of a transistor is equal to the sum of the base and collector currents. Thus, since the magnitude of the emitter current is constant, as the magnitude of the base current decreases, the magnitude of collector current increases with increased collector-to-base voltage, for instance.